Room Acoustics

Room acoustics and how to treat a room is a HUGE topic,
this page only a scratches the surface of Room acoustics as there
are so many different variables that effect sound. Only the main topics
will be discussed here and should form a good starting point for learning
what you want in this topic. This page concentrates mainly on improving
the sound of a room, NOT sound proofing a room which is completly different.
By sound proofing a room, your not only keep the sound out, you also keep
sound in the room which can cause problems such as large RT60 and slap
echos. The following covers these topics and will help you to correct
problems that rooms and sound proofing cause. This is written for both
small and large rooms which may be used for recording.

What makes a room sound good ?

Only one thing makes a room sound good. REVERB. The reverb of a room
needs to be equal right across the frequency spectrum for this to be true.
If the top end dies away really fast, yet the bottom end or the mids hang
around twice as long the reverb may destroy the room sound. Also too much
reverb in a room even if its balanced reverb can destroy the intelligibility
of spoken words or faster paced music. I'm sure you've starting singing
in the bathroom before and realized how much better your voice sounds
in a room with REVERB. A room without reverb does not sound very nice,
most people would agree on this.

What is Reverb and RT60?

Reverb is made up of heaps of echo's/reflections off hard and reflective
surfaces. Each walls reflections combine to make up a smooth tail to a
sound called REVERBERATION. If you are speaking and the sound carries
on after you have stopped speaking, that's reverb at work. A nasty form
of reverb is the ECHO which is not smooth. A particularly nasty type of
echo is the FLUTTER ECHO, which is mainly caused by parallel reflective
walls. RT60 is the term given to the length of reverb. If the sound takes
3 seconds to die down to 1 millionth of the original intensity (-60dB)
, then the RT60 of the reverb is 3 seconds.

Churches and reverb

Traditional churches were acoustic nightmares (for fast paced music and
spoken word) as the room was designed to amplify and have very long reverb
times. Choirs on stage would sound 3 times as large with larger reverb
times, hence the design of cathedrals. This reverb sounds great on the
choir, but on the spoken word the intelligibility is greatly effected.
The congregations struggles to hear the words and will very quickly become
fatigued. Modern churches with drum kits and full rock bands that play
very fast also suffer from the long reverb as each beat of the sound carries
well into the next or even 6 beats time. Hence the reason why many church's
are using acoustic treatments to control the sound. The following will
help churches with acoustic problems greatly as large RT60's are normally
found in larger rooms. Its much cheaper to design a good room from the
start then try to fix a problematic room.

Building a good room

To build a good room for sound there are three main things to take into
account. First is that Bass takes space to correctly form (at least 1/4
of a wavelength is needed), so smaller rooms should have high ceilings
just like early church's. Secondly the dimension of a room should NOT
be a multiple of the same number...eg. 20 foot by 30 foot by 10 foot high.
A room with these dimensions would have serious problems as a common frequency
or ROOM MODE would become very prominent and cause feedback as the room
will become RESONATE around this frequency and harmonics. Lastly the room
should not have or as few as possible parallel walls/roof. 12 degrees
offset is normally sufficient that eliminating these room modes from occurring
as the walls are not a set distance apart. 6 degrees for each wall is
not a lot to offset and can fix a lot of standing waves not to mention
the reflections.

How do I fix excessive echo/reverb, and
make a room sound better ?

If a room has a problem with echo's you will find that if you treat only
one wall of a room which you think is a problem, the others walls that
are untreated will become more apparent. So treating all walls/roof/floor
is necessary to combat echo problems with absorbtion, which will oversuck
a room and leave it dead. If you dont want a dead room then you can fix
with two methods through scattering the sound called DIFFUSION and secondly
through ABSORPTION. Remember how above I stated that smooth and equal
reverb sounds good in a room ? Well most people when treating a room will
do one of two things. The first is to concentrate on absorption and suck
all the reverb/life out of the room. Anecoic chambers are rooms used for
testing sound equipment mainly microphones and speakers. These rooms have
zero reflections through the use of BLACK EYE's and absorption over all
walls. If someone who is not used to working in such an environment is
placed in a room with no reverb they will feel very strange. Hearing is
used for balance and sense of direction and without reverb and reflections
the human body feels very weird. This is taking it to an extreme, but
still proves the point that absorbing too much sound out of a room is
going to make people feel uncomfortable and make the room feel unwarm.
The second thing that people do wrong is only absorb half the sound. By
this I mean only the top end (treble) is absorbed and the bottom end (bass)
still has a large RT60 making the room appear dead but still have a BOOM
to it. Making the reverb in a room more smooth and equal through diffusion
and some absorption in problematic frequencies will result in the best
acoustic treatment for that room. Going back to to what makes a room sound
good, you first need to listen to the room and decide if the room echo's,
or has a smooth reverb tail. Secondly you need to then look at the reverb
time of the top end and the bottom end. If the room is small and has hollow
plaster walls then the room wont/may not have problems with the bottom
end and Diffusion will be the best way to improve the rooms sound. The
hollow walls can create a bass trap and the small room dimensions mean
small RT60. The rooms echo's will be smoothed out into reverb by the scattering
of the sound off diffusion panels. The second common type of room is the
large concrete room that has solid walls. This type of room will need
bottom end absorption to stop the BOOMiness of the room and diffusion
to smooth out the sound. A large room may also have too much reverb so
some full spectrum absorption should be added to reduce the RT60 of the
room. The absorbtion should be scattered around the room and not just
on one wall. 10 - 40 % coverage of total wall space is normally enough.

To sum up you need to turn echo's into reverb with diffusion panels,
and secondly you need to ensure that the bottom end had the same RT60
as the top end. After achieving those two things you can use broadband
absorption to reduce the overall reverb time (RT60) spreading the absorbtion
over all the walls not just one. This is hard to get right and why professionals
are normally called in for advice who can calculate how much of each type
of treatment is needed in a room.

Voice over rooms and ISO
booths

This type of room is best when heavy use of broadband absorption all
around is used. More than 70% coverage of the rooms walls and roof should
be covered in a absorbing material. The floor doesn't matter that much
as the human ear cant hear as well from below.

Drum rooms

Drums sound best in a live/reflective environment A wooden room tends
to bring out the best in a drum kit and for that matter most acoustic
instruments. The room contributes HUGELY to the sound of a drum kit and
using absorbing materials in a drum room will destroy the sound. In this
type of situation if the room is small to mid sized the best treatment
is using reflective diffusion panels to smooth out the rooms sound.

If you look over the chart you see a few patterns forming. One is that
for low end absorbtion you need thicker layers than you do for high end
absorbtion. 4 inches or more of fibreglass batts (PinkBatts), Rockwool
or my favourite the "Fibreglass Wool Rigid Board" is probably
the best broadband absorbtion you can use. If you look at the chart linked
above you see the even absorbtion of Fibreglass over all frequencies,
which is what you want unless the room needs special attention of certain
frequencies which is where this chart helps out heaps. Find out what frequencies
are the worst in a room and treat it with a material to suit. Bass Traps
are designed to only absorb the bottom end in a room leaving the top end
untouched. Traps are worth leaving to an expert. Bass and Low mid traps
are very commonly used in Mastering studios to treat the room and reduce
any problem areas.

I may add Bass traps later on ......

Further reading.....

The two best sites I have found on these topics and sound proofing are
....